Soundboard crown

[Delwin D Fandrich]

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Re: Soundboard crown
  ----- Original Message -----=20
  From: Phillip Ford=20
  To: Pianotech=20
  Sent: August 17, 2003 1:53 PM
  Subject: Re: Soundboard crown




      ----- Original Message -----
      From: Erwinspiano@aol.com
      To: pianotech@ptg.org
      Sent: August 16, 2003 9:29 AM
      Subject: Re: Soundboard crown



      >>Phil-- My thought was that any wood will shrink over time =
especially quartered wood because of it's expansion contraction =
properties and actually that could take some time.=20
    Wood does not shrink on its own--at least not so you would =
notice--only as a result of changing environmental conditions, i.e., =
with changes in the MC of the wood. Kept in an temperature/humidity =
stable environment what would cause it to shrink?

  I was thinking that normal seasonal variations in humidity would cause =
it to shrink.
I was refering to the first comment, above. The reduction in panel width =
that takes place because of compression set is not technically shrikage. =
It is a reduction in size resulting from compression damage to the wood =
cells.=20


    I assume you're referring to the (so far, unsubstantiated) phenomena =
of "oil-canning."


  I don't think I was.  But I'm not really sure what that is.  Can you =
explain what you mean by this?
The effect noted when depressing the bottom of an oil can. Initially it =
is bowed out slightly. By pressing on it it pops in to approximately the =
same radius in the reverse despensing a more-or-less precise amount of =
oil in the process.


  I wasn't really referring to a compression crowned board.  I think the =
configuration that I have in mind would be uncrowned or reverse rib =
crowned, if that makes sense.  I wasn't thinking of a CC board that had =
collapsed.  I was thinking more of a board that was made deliberately =
made to have a reverse crown, such as some of the pianos that Dale has =
seen (assuming they were intended to have reverse crown).  So, take an =
undried panel, glue on some ribs, and load it up.  It will reverse crown =
and the board will be in tension I think.  Or machine crown some ribs to =
give the board a reverse crown when they are glued on.  Load it up and =
once again I think the board will be in tension.  Now if this board is =
subjected to a drier environment that that at which is was glued up what =
will happen?  I was assuming that the board would take on additional =
tension.  Enough to overload it?  I don't know.  But if not, then when =
this board is subjected to more humid environments than that at which it =
was glued up it would probably be less likely to sustain compression =
damage than a CC board, or even a rib crowned board with positive crown.
Yes, I see what you mean. I don't see how, in anything remotely =
resembling average climate conditions, you would generate enough tension =
to cause the thing to crack. Assuming the panel was at some MC in the 7% =
to 9% range when it was ribbed.

I still think, however, if the board were taken down to typical =
compression-crowning MC levels before ribbing the effect of the =
developing stress interface would work to force the board into a =
positive crown.=20

The propensity of any soundboard to develop cracks depends on its MC =
when glued to the ribs. If a panel is glued up at 3.8% to 4.0% MC even =
in a reverse crown configuration it is going to develop significant =
compression as it takes on moisture. How could it avoid doing so? It is =
the extremely low initial MC and the act of gluing the thing to those =
perpendicular-to-grain ribs that creates the problem.

It will be interesting to read about the results of your experiment, =
however. When will you be finished?

  I was also speculating about what would happen to this board if it was =
subjected to a drier or more humid environment.  My thought was that in =
a drier environment the board would want to move up.  My thought was =
that the board is describing an arc.  As it dried this arc would have to =
get shorter, so the board would want to flatten out, which in this case =
would mean moving up.  You seem to think it would move down.  Why do you =
think that?
Good question. You'll have two forces fighting each other, but they'll =
not be equal forces. I think the stress interface will win. At least =
until the panel self-destructs.

I think the shrinking panel will crack before it creates enough lifting =
force via tension to accomplish anything and the stress interface will =
pull it down. As well, I think an expanding panel will work against =
itself but the stress interface will create the enough force to push the =
ribs up no matter how those ribs are initially crowned. If the ribs are =
machined with a reverse crown and glued to a panel with very low MC, the =
expanding panel will still want to force the assembly positive. It may =
not make it but it will sure try.

Will this be part of your experiment?


    With a conventional board having crown and with a downbearing load I =
think the board is always in compression.... =20

    I am uncertain what is meant by "a conventional board." Assuming =
this refers to a compression-crowned soundboard system, the key words in =
the above are then "having crown." However, a strong argument can be =
made that this is no longer--if it ever was--the conventional soundboard =
system....


  I wasn't intending to comment on CC vs RC boards here.  By =
conventional, I meant a board that was intended to to have a positive =
crown, which includes every piano I've ever seen (but not every piano =
that Dale's ever seen).  I therefore felt safe in calling this =
'conventional'.
Ah, but that doesn't correlate to the first statement above which =
states, "With a conventional board having crown and with a downbearing =
load I think the board is always in compression." This is the comment I =
was responding to and wondering just what was meant by "conventional." =
Over the decades I suspect far more pianos have been built with either =
hybrid systems (probably the most common) or rib-crowned system than =
have been built using compression-crowned systems. And both the hybrid =
and the pure, or nearly pure, rib crowned board will have crown even =
without any compression in the panel. Our panels are ribbed at 6.5% MC. =
When atmospheric conditions are such that the panel is at 6.5% MC there =
is no internal compression within the panel. Yet there is design crown. =
Over the years if these boards are subjected to very high levels of =
humidity (and the corresponding high MC) they will also undergo some =
amount of compression set. Still, there will be crown when they return =
to equilibrium at whatever MC that may end up being.


   I was pondering a what if scenario.  What if you built a board that =
was intended to have a reverse crown.  How would it behave?  Would it =
have any advantages over a 'conventional' board that is intended to have =
positive crown?

    As well, it is quite possible to have a string downbearing load =
without having positive crown.

  Yes, I know.  But in this case it would be a deliberate design feature =
rather than an unintended consequence.
Well, there is always the Rippen--the only modern piano I know of built =
deliberately to have reverse crown. Actually, the (laminated) boards =
were ribbed flat and, when loaded with string bearing, were forced to a =
reverse crown. They worked. At least within the limitations of their =
less than desirable scaling. As to whether they might have worked better =
with some form of positive crown, I have no idea. Nor do I have any idea =
how they would have worked had their panels been made of solid stock.

Del 
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